The present invention relates to a semiconductor laser device and a fabrication method therefor, and, more particularly, to a semiconductor laser device which can be used preferably as a light source for an optical disk apparatus or the like and a fabrication method therefor.
In a semiconductor laser device, particularly in an AlGaInP semiconductor laser device in which the heterojunction between an active layer and a clad layer cannot have a sufficiently high barrier, a technology for controlling impurity doping has an important influence on the characteristics of the device.
Specifically, there has been conventionally known an AlGaInP semiconductor laser device wherein the clad layer adjacent to the active layer is left undoped by controlling the position at which impurity doping is performed during the fabrication thereof. The conventional AlGaInP semiconductor laser having such a structure is advantageous in that the diffusion of the impurity from the clad layer into the active layer can be prevented.
However, the conventional AlGaInP semiconductor laser device described above has the problem that, when the position at which impurity is performed is further away from the active layer, the efficiency of conversion from an injected current to light deteriorates so that the operating current thereof increases. Conversely, when the position at which impurity doping is performed is closer to the active layer, the problem occurs that the diffusion of the impurity reaches the interior of the active layer due to a thermal process or the like and the deterioration of the device occurs within a short time of about several hours to reduce the lifetime of the device and consequently degrade the reliability of the device.
Thus, in the conventional AlGaInP semiconductor laser, it has been necessary to control impurity doping with high accuracy so that considerable difficulty has been encountered in the fabrication thereof.
To reduce the difficulty, Japanese Laid-Open Patent Publication No. HEI 11-87831 proposes a semiconductor laser device having a structure which can not only prevent the diffusion of an impurity into the active layer but also improve light emission efficiency, reduce the operating current, and suppress the reliability degradation of the device by controlling the lattice mismatch of the clad layer with respect to a semiconductor substrate.
However, the semiconductor laser device having the structure which controls the lattice mismatch of the clad layer with respect to the semiconductor substrate described above has had the problem that a crystal defect occurs in the active layer and the long term reliability of the device cannot be guaranteed, though the impurity diffusion into the active layer can be prevented. In addition, the semiconductor laser device also has had the problem that the setting of process conditions in forming a ridge waveguide is difficult and the production yield thereof is low.